Reforming embraces several reactions, such as dehydrogenation, isomerization, dehydroisomerization, cyclization and dehydrocyclization. In the process of the present invention, aromatics are formed from the feed hydrocarbons to the reforming reaction zone, and dehydrocyclization is the most important reaction.
U.S. Pat. No. 4,104,320 to Bernard and Nury discloses that it is possible to dehydrocyclize paraffins to produce aromatics with high selectivity using a monofunctional non-acidic type-zeolite L catalyst. The zeolite L based catalyst in '320 has exchangeable cations of which at least 90% are sodium, lithium, potassium, rubidium or cesium, and contains at least one Group VIII noble metal (or tin or germanium). In particular, catalysts having platinum on potassium form L-zeolite exchanged with a rubidium or cesium salt were claimed by Bernard and Nury to achieve exceptionally high selectivity for n-hexane conversion to benzene. As disclosed in the Bernard and Nury patent, the zeolite L is typically synthesized in the potassium form. A portion, usually not more than 80%, of the potassium cations can be exchanged so that other cations replace the exchangeable potassium.
Later, a further important step forward was disclosed in U.S. Pat. Nos. 4,434,311; 4,435,283; 4,447,316; and 4,517,306 to Buss and Hughes. The Buss and Hughes patents describe catalysts comprising a large pore zeolite exchanged with an alkaline earth metal (barium, strontium or calcium, preferably barium) containing one or more Group VIII metals (preferably platinum) and their use in reforming petroleum naphthas. An essential element in the catalyst is the alkaline earth metal. Especially when the alkaline earth metal is barium, and the large-pore zeolite is L-zeolite, the catalysts were found to provide even higher selectivities than the corresponding alkali exchanged L-zeolite catalysts disclosed in U.S. Pat. No. 4,104,320.
These high selectivity catalysts of Bernard and Nury, and of Buss and Hughes, are all "non-acidic" and are referred to as "monofunctional catalysts". These catalysts are highly selective for forming aromatics via dehydrocyclization of paraffins.
Having discovered a highly selective catalyst, commercialization seemed promising. Unfortunately, that was not the case, because the high selectivity, L-zeolite catalysts did not achieve long enough run length to make them feasible for use in catalytic reforming. U.S. Pat. No. 4,456,527 discloses the surprising finding that if the sulfur content of the feed was reduced to ultra low levels, below levels used in the past for catalysts especially sensitive to sulfur, that then long run lengths could be achieved with the L-zeolite non-acidic catalyst. Specifically, it was found that the concentration of sulfur in the hydrocarbon feed to the L-zeolite catalyst should be at ultra low levels, preferably less than 100 parts per billion (ppb), more preferably less than 50 ppb, to achieve improved stability/activity for the catalyst used.
It was also found that zeolite L reforming catalysts are surprisingly sensitive to the presence of water, particularly while under reaction conditions. Water has been found to greatly accelerate the rate of deactivation of these catalysts. U.S. Pat. No. 4,830,732, which is herein incorporated by reference discloses the surprising sensitivity of zeolite L catalysts to water and ways to mitigate the problem. U.S. Pat. No. 5,382,353 and U.S. Pat. No. 5,620,937 to Mulaskey et al., which are herein incorporated by reference, disclose a zeolite L based reforming catalyst wherein the catalyst is treated at high temperature and low water content to thereby improve the stability of the catalyst, that is, to lower the deactivation rate of the catalyst under reforming conditions.
During commercialization of zeolite L reforming catalysts, it was found that the ultra low sulfur levels caused the unexpected problem of coking, carburization and metal dusting of the reactor system metallurgy. This problem has necessitated the use of special steels and/or steels having protective layers to prevent coking, carburization and metal dusting. When used, protective layers are provided on the steel surfaces that are to be contacted with hydrocarbons at process temperatures, e.g., at temperatures between about 800-1150.degree. F. For example, a tin protective layer has been used in the reactors and furnace tubes of a catalytic reformer operated at ultra low sulfur levels. This has effectively reduced the rate of coke formation exterior to the catalyst particles in the reactors. Without this protection, coke buildup would have resulted in massive coke-plugging and in reactor system shutdowns. These problems are described in detail in Heyse et al., U.S. Pat. No. 5,674,376. Heyse et al, disclose the use of special steels and protective coatings, including tin coatings, to prevent carburization and metal dusting. In a preferred embodiment, Heyse et al., teach applying a tin paint to a steel portion of a reactor system and heating in hydrogen to produce a carburization-resistant intermetallic layer containing iron and nickel stannides. The reforming system of Heyse et al., is a high temperature, low sulfur and low water system that uses a conventional reformer designs, i.e., furnaces for heating the feed and catalysts located in conventional reactors.
Recently, several patents and patent applications of RAULO (Research Association for Utilization of Light Oil) and Idemitsu Kosan Co. have been published relating to use of halogen in zeolite L based monofunctional reforming catalysts. Such halogen containing monofunctional catalysts have been reported to have improved stability (catalyst life) when used in catalytic reforming, particularly in reforming feedstocks boiling above C.sub.7 hydrocarbons in addition to C6 and C7 hydrocarbons. In this regard, see EP 201,856A; EP 498,182A; U.S. Pat. No. 4,681,865; and U.S. Pat. No. 5,091,351.
EP 403,976 to Yoneda et al., and assigned to RAULO, discloses the use of fluorine treated zeolite L based catalysts in small diameter tubes of about one-inch inside diameter (22.2 mm to 28 mm in the examples). Heating medium proposed for the small tubes were molten metal or molten salt so as to maintain precise control of the temperature of the tubes. Accordingly, EP 403,976 does not teach the use of a conventional type furnace or conventional type furnace tubes. Conventional furnaces for catalytic reforming have tubes of usually three or more inches in inside diameter (76 mm or more), whereas EP 403,976 teaches that using tubes having an inside diameter greater than 50 mm is undesirable. Also, conventional furnaces are heated using gas or oil fired burners.
Typical catalytic reforming processes employ a series of conventional furnaces to heat the naphtha feedstock before each reforming reactor stage, as the reforming reaction is endothermic. Thus, in a three-stage reforming process, the overall reforming unit would comprise a first furnace followed by a first-stage reactor vessel containing the reforming catalyst (over which catalyst the endothermic reforming reaction occurs); a second furnace followed by a second-stage reactor containing reforming catalyst over which the reforming reaction is further progressed; and a third furnace followed by a third-stage reactor with catalyst to further progress the reforming reaction conversion levels.
For example, U.S. Pat. No. 4,155,835 to Antal illustrates a three-stage reforming process, with three furnaces (30, 44, 52) and three reforming reactors (40, 48, 56) shown in the drawing in Antal. Example reforming reactors used according to the prior art arc shown, for instance, in U.S. Pat. No. 5,211,837 to Russ et al., particularly the radial flow reactor shown in FIG. 2 of Russ et al.
In some catalytic reforming units, as many as five or six stages of furnaces followed by reactors are used in series for the catalytic reforming unit. In particular, reforming of hydrocarbons over a Pt L zeolite catalyst is a highly endothermic reaction and can require as many as 5 or 6 stages or more of furnaces followed by reactors. The present invention allows such a multistage process to be greatly simplified to two, or more preferably one, furnace reactor.